1. Field of the Invention
This invention relates to a process for preventing growth of fishtails during slabbing, and more particularly to a method of reducing crop loss caused by the growth of fishtails produced at longitudinally opposite end portions of the top and bottom of a material to be rolled during slabbing.
2. Description of the Prior Art
Most of steel ingots, which have been cast in a steel works, are charged into a soaking pit as in a hot state, and slabbing-rolled to provide slabs and blooms.
During this slabbing, at the opposite end portions of the top and bottom of the steel ingot, there are grown fishtails 2 as shown in FIG. 1 and also overlaps 4 each consisting of an upper and a lower sheets, thus resulting in crop loss. Since the crops are shorn off to provide a good product, the amount of shorn-off crops adversely affects the yield of slabbing to a considerable extent. Hence, there have heretofore been proposed various methods for preventing the fishtails and overlaps from growing.
As one of the method of preventing fishtails from growing, there has been known a method wherein a strong reduction for reducing the width of a slab, which is a so-called edging, is practised at a stage where the thickness of the slab in cross section becomes fairly thin. However, during slabbing, there has generally been effected a comparatively strong reduction during a first pass for the purpose of removing scales off the surfaces of the steel ingot. Since the beginning of growth of fishtails during this first pass considerably adversely affects the final shapes of crops at the end of rolling, the conventional method, wherein a strong edging is effected at the final stage of rolling, is not effective in preventing the fishtails from growing.
On the other hand, although the reduction of the overlaps is mostly achieved by the method of preventing fishtails from growing, it is deemed preferable that, in rolling the steel ingot in the thicknesswise direction thereof, both the reduction value and the reduction ratio are increased per pass. However, as for the reduction value per pass, generally restrictions are placed on the adoptable reduction value due to the conditions of the limits imposed on the bite-in angle, torque and load to be applied during rolling, varying from the capacity of the rolling mill, the deformability of the material and the like.
From the reasons as described above, no radical countermeasures for reducing overlaps have been taken so far.
Consequently, in the present circumstances, there has not been adopted any effective method for reducing crop loss at the opposite ends of the steel ingot, which is caused by the growth of fishtails and overlaps during slabbing.